1. Field of the Invention
The proposed technical solution falls within the field of altering physical properties of steels by means of forming.
2. Description of the Related Art
TRIP steels are high-strength multiphase steels that contain ferrite, bainite and retained austenite. They have been developed for making sheet parts in automotive industry. However, their large capacity for deformation makes them suitable candidates for other processes as well. Cold forming used for achieving the required shape of the part is one of such processes. During cold deformation, retained austenite transforms into martensite, after which TRIP steels were named: Transformation Induced Plasticity.
These materials are well established in production of steel sheet. There are two basic methods of their processing. The first relies on hot rolling of sheets (represented by the wavy line located above Ac3 on the isothermal transformation curve of the temperature (T) vs. time (t) chart for steels) in fully austenitic condition followed by cooling down to the bainite nose area (FIG. 1) (in the isothermal transformation curves of FIGS. 1, 2 and 3, the curve F represents the ferrite formation nose, the curve P represents the pearlite formation nose and the curve B represents the barite formation nose). A hold at that temperature causes a certain proportion of metastable austenite to decompose into bainite. The remaining part of retained austenite is preserved. Upon the hold, the retained austenite remains stable enough to survive further cooling to room temperature.
The second method uses hot forming (represented by the wavy line on the isothermal transformation curve located above AC3) followed by cold forming (represented by the wavy line on the isothermal transformation curve located below MS) (FIG. 2). The resulting metal sheet is annealed in the intercritical region between Ac1 and Ac3. This leads to incomplete austenitization. The material is then cooled down to and held at the bainite nose temperature in order for bainite to form and for retained austenite to become stable. Both of the above-described methods lead to multi-phase microstructures containing ferrite, bainite and retained austenite.
The drawback of hot forming lies in that the material is heated to the fully austenitic region, i.e. its temperature is relatively high above Ac3. The surface at this temperature oxidizes rapidly. Scales impair the surface quality and cause materials losses. In addition, heating of feedstock to high temperatures requires relatively large amount of energy.
These drawbacks are substantially alleviated by the proposed solution.